Injection molding systems generally include an injection molding unit and a hot runner system, wherein the hot runner system has one or more manifolds and one or more nozzles in fluid communication therewith. More particularly, the hot runner manifolds receive a melt stream of moldable material from the injection molding unit and transfer the melt stream to one or more mold cavities via a respective hot runner nozzle. Generally, hot runner systems offer the choice between nozzles that are thermal gated or valve gated. Valve-gated nozzles are generally used in molding applications where the esthetic appearance of the molded part is important, because such nozzles usually provide a better gate vestige on the molded part over thermal gated nozzles.
Each valve-gated nozzle has a valve pin extending from a valve pin actuator or a valve pin may be coupled to an actuatable valve plate. In either case, the valve pin passes through the manifold and extends into and through a melt channel of the nozzle to have a downstream end seatable within a mold gate of a mold cavity. Control of the melt stream is achieved by raising and lowering the valve pin. More particularly, retracting the valve pin from the mold gate permits the melt stream to flow into the mold cavity while re-seating the downstream end of the valve pin within the mold gate prevents further flow of the melt stream into the mold cavity. The movement of the valve pin is controlled by the valve pin actuator, which may be, for example, a pneumatic or hydraulic system that raises and lowers the valve pin.
The intermittent flow of the melt stream through the manifold and nozzles, the cooling of the mold to effect removal of the molded parts and the subsequent re-heating and control of the temperature for injection of moldable material, results in temperature changes in the valve pins, the nozzles, and the manifold. The required temperatures and control of such temperatures are achieved using heaters in the manifold and/or nozzles.
The heating, cooling and temperature control of the manifold, nozzles, and mold plates of the injection molding system, including during start up and shut down of the injection molding system, results in some thermal expansion and contraction of the hot runner components. Relative changes in position of the manifold and nozzle assemblies caused by the thermal expansion and contraction apply stress to the valve pins, which pass through the manifold and extend into the nozzles, with the consequence that the valve pins tend to be forced away from alignment with the mold gates, i.e., are deflected. This can lead to bending and damage of the valve pins which in turn can result in damage to the mold gates. Uncontrolled bending of the valve pins and damage to the mold gates can lead to loss of control of flow of the melt stream through the mold gates, for e.g., by poor seating of the downstream ends of the valve pins in the mold gates and/or changes in timing of closing of the mold gates. This in turn may lead to processing problems, such as melt drool at one or more of the mold gates or inconsistent injection of the melt stream into the mold cavities.
Some prior solutions to accommodate thermal changes in an injection molding system include bolting or otherwise securing the valve-gated nozzle assemblies and manifold together, so that there is no relative movement in the positions of the valve pin actuators, manifold and nozzles with a change in temperature. However, this can lead to stress on the valve pin actuators due to the high temperature of the manifold as well as in other parts of the injection molding system and may create increased heat loss. Alternatively, the manifold may be allowed to float in position with respect to both the valve pin actuators and the valve-gated nozzles. In other words, the manifold is not bolted to the valve pin actuators or the mold plate within which they are seated or to the valve-gated nozzles, in which case there is a tendency for the valve pins to be deflected from alignment and become distorted or bent as the manifold undergoes thermal expansion and/or contraction.
It is therefore an object of the present invention to provide a novel solution for accommodating thermal expansion and contraction in an injection molding system that utilizes valve-gated nozzles that also addresses one or more of the deficiencies noted above.